Background and significance: Evidence-based psychotherapies (EBP) for PTSD, such as Prolonged Exposure (PE) and Cognitive Processing Therapy (CPT), are effective, but only a minority of Veterans initiate and engage in these treatments. Common Veteran concerns about starting these therapies include lack of emotional readiness and skepticism of the treatment rationale. Additionally, up to 60% of Veterans continue to meet diagnostic criteria for PTSD after completion, and even those who no longer meet criteria may have remaining psychosocial needs as they rebuild their lives. Peer support programs have the potential to improve patient outcomes by facilitating treatment initiation, engagement, and aftercare. In peer programs, Veterans recovering from PTSD provide emotional support, empathy, and information based upon their own lived experience with PTSD. Guided by social learning theory, peers can model healthy coping behaviors like confronting trauma directly, engaging in therapy, and rebuilding a fulfilling life during the process of recovery. Peer support is well-established for other disorder and is a priority within the VA. It supports treatment initiation, improves functioning and qualityof life, reduces utilization of mental health services, and is highly acceptable to Veterans. Despite the recent expansion of peer programs in VA, more research is needed to examine the most effective ways to integrate peers with existing PTSD care to support evidence based treatments across the continuum of PTSD care. The goal of this CDA is to refine and pilot test a peer support program for PTSD, with separate components for initiation/engagement of EBP and aftercare. Research Plan: In Aim 1, through an iterative formative evaluation with stakeholders (Veterans, peer providers, PTSD providers, and national leaders), and based upon the prior needs assessment, we will refine a peer support program that addresses treatment initiation and engagement. Because community based outpatient clinics (CBOCs) have fewer resources to support PTSD care, we will target these settings, as well as a VAMC, for the peer program. In Aim 2, we will conduct an open feasibility pilot for the peer support initiation/ engagement groups to further refine this program, and then test effectiveness in a pilot randomized trial. Primary outcomes will be EBP attitudes, initiation and completion of EBP. These pilot data will support an IIR submission for a Hybrid Type I Effectiveness/Implementation trial in year 3. In Aim 3, we will conduct a formative evaluation to explore stakeholders' opinions of a proposed aftercare peer program for Veterans who complete EBP. In Aim 4, we will pilot feasibility of the peer program as aftercare in two peer support groups and collect qualitative data to guide revisions. Primary outcomes will be functioning and quality of life. In year 5, we will submit an HSR&D Pilot to conduct a pilot randomized trial of peer support as aftercare. Career Plan: My career goal is to become an expert in testing and implementing novel care delivery methods for PTSD in VA. My dedication to the VA, research on PTSD, foundational training in HSR, and expertise using nontraditional mental health interventions make me ideally positioned to conduct this research. During the CDA period, training will focus on providing me with advanced health services training, including methods of formative evaluation for program development, methods and analysis of complex effectiveness-implementation designs to test novel delivery models, and skills to obtain HSR funding to continue this program of research. This will occur through academic courses, conferences, experiential training in conducting the proposed research, and serving as a co-investigator on HSR&D projects. Further guidance will be provided by a mentorship team composed of senior health services researchers with expertise in program development, implementation, PTSD and peer support, and qualitative and quantitative methods.